Mechanical face seals are used to control leakage from pumps, mixers, agitators, and the like. Seals are among the most crucial components of industrial machinery. If a seal fails prematurely, it can have significant economic and environmental consequences.
A basic mechanical seal is a mechanically loaded device consisting of a rotating (primary) ring and stationary (mating) ring, having lapped faces that operate in close proximity under hydraulic pressure from fluid containment as well as the spring force that pushes the rings together to minimize the leakage between the rotating shaft and the stationary housing. A common material used in primary rings is carbon graphite, and ceramic, stainless steel, tungsten carbide and silicon carbide are popular materials for use in mating rings. Coolant (flush fluid) may be supplied to lubricate and remove heat from the interface between the two rings.
Most mechanical seals fail long before they wear out, with high temperatures identified as some of the main causes of their failure. Heat is generated at the interface as the primary ring rubs against the mating ring during operation. Too much heat can cause thermal distortions on the seal face and accelerate wear, and thus increase the leak path. Further, heat effects are known to be responsible for thermo-elastic instabilities (TEI) that occur due to high speeds and high loads, particularly if the seal material is prone to heat checking. These instabilities give rise to the formation of macroscopic hot spots on the seal faces interface. Hot spots may expand relatively more than adjacent areas, causing higher local pressures that act on the surface and generate more frictional heating. This is analogous to a positive feedback loop in a control system, causing thermally induced instability.
Both conduction and convection heat transfer play a significant role on the performance of a mechanical seal. Since heat conduction occurs as heat flows through the primary ring and mating ring, the thermal conductivity of these materials is important. In addition, heat generated at the interface between the mating and rotating ring is dissipated into the flush fluid through the process of convective heat transfer. In order to remove the heat generated at the faces very quickly, a high heat transfer coefficient and/or a larger wetted area is needed. Heat transfer from seal face is mostly dissipated through axial and radial directions. Therefore, increasing the wetted area in the axial and radial directions can be considered for improving heat transfer. However, an increase in the surface area is not always possible due to space and/or design limitations. Therefore, new heat transfer augmentation techniques are needed to reduce interface temperature.
There are many heat transfer augmentation techniques employed in the engineering field, such as pin fins, rib turbulators, and dimpled surfaces. Dimples may be defined as pits, bores, holes, or any other depressions formed into a surface. Dimples can be easily fabricated using such techniques as a laser engraving machine, which can quickly “burn” textures on different material from carbides to metals, or any other suitable technique. Arrays of surface dimples are used in wide variety of practical applications such as electronics cooling, heat exchangers, turbine blade internal cooling passages, etc. However, most of studies on utilizing dimpled surfaces in mechanical seals consider turbulent flows at high Reynolds numbers while studies pertaining to thermal performance for dimpled surfaces in laminar flows are, quite rare.
Some studies have examined the placement of dimples on the active surface of the mating ring of a mechanical seal. While such placement may achieve a desired effect on the friction generated between the mating ring and the stationary ring, texturing the active surface of the mating ring may create an additional load-carrying capacity with each dimple such that the summation of all of them could created a lifting force. Such a lifting force may separate the faces and thus create a leak path and thus render the seal ineffective. Further, considering a seal in which the active surface has been lapped to within 2 helium light bands (a common practice), adding dimples to the active surface would require a second lapping step after the dimples are added, thus introducing a greater expense and complexity to the manufacturing process. Additionally, other recent studies suggest that adding dimples or otherwise texturing the active surface of the mating ring may lead to increased wear.
Other recent studies regarding the use of dimple textures in mechanical seals are associated with an internal cooling process whereby the coolant is pumped through small channels with dimpled surfaces. However, such designs likely require potentially drastic changes to existing seal configurations, the flush plan, and/or the use of additional auxiliary parts in order to accommodate the internal cooling process. Accordingly, there is currently an unfilled need for a mechanical seal with superior heat transfer and wear characteristics that nevertheless functions with conventional mechanical seal configurations.